That's how they come from the factory. Go to your local hardware store and get a wax ring for a toilet and melt in on there with a torch.
Umm...... You're not pointing the flame directly at the tank. You hold the wax ring above the sending unit and let it drip onto it. I've done it 3 times without killing myself...Diesel doesn't explode anyway, it just burns.?????? An open flame to help seal a fuel tank opening??? Please tell me this is an April Fool's Day post!
The rubber O-ring seal for the fuel tank sending unit is replaceable and available at most parts stores. The white wax-like substance is most likely lithium grease applied to keep the retaining ring from tearing the rubber as it's rotated into the lock position. I lubricated mine with some liquid dish soap. ~FH